1. Field of the Invention
The present invention relates in general to electric motors for driving an automotive blower, and more particularly, to the electric motors of a low-noise type. More specifically, our present invention is concerned with a low-noise motor assembly for an automotive blower, being installed therein a rotation speed controller including a pulse width modulating device.
2. Description of the Prior Art
An automotive blower comprises generally a fan, an electric motor for driving the fan and a speed controller for controlling the rotational speed of the electric motor. Among speed controllers hitherto employed, there is a type which uses a power transistor for functional and commercial superiority, in which the control of the rotation speed can be made in a stepless manner. For dealing with the considerable heat generated by the power transistor, the transistor is mounted to a heat sink which is located in a well-ventilated area. In fact, in case of an automotive air conditioner, the heat sink is exposed to an air passage defined in an air distributing unit of the air conditioner.
Some of the speed controllers of this power transistor type are shown in Japanese Patent First Provisional Publication No. 60-148,154 and Japanese Utility Model First Provisional Publication No. 63-153,542.
Recently, the field of automotive blowers shows a tendency to increase the air blowing capacity due to the need for supplying air to various areas a motor vehicle. While, in the field of the motor vehicles, there is a tendency to increase the number of electrically operated parts for easy with which the parts are manipulated by users.
However, these tendencies cause the power shortage of batteries and alternators mounted in the vehicles.
One of measures for solving this power shortage is to use a pulse width modulating device for controlling the rotation speed of the electric motor for the blower. In fact, this measure has induced a power saving in the electricity consumed in the motor vehicle. However, this measure has brought about a problem of radio noise caused by the pulses generated by the PWM (pulse width modulating device). Thus, hitherto, such measure has not been put to practical use.